The invention relates to coated metal peroxides, their preparation and use.
Metal peroxides, in particular magnesium, zinc and calcium peroxide or mixed peroxides, are known from the prior art and are used as oxygen donors in numerous applications. Thus alkaline earth peroxides are used for medicinal and pharmaceutical purposes and in cosmetics. Furthermore, the use of the peroxides in toothpastes, for producing bread or for seed preparation is known. The possibility of the slow emission of oxygen of peroxide compounds is also utilized in sewage treatment or in the purification of soil.
Alkali and alkaline earth peroxides are generally prepared by preparing peroxide-containing reaction mixtures from aqueous solutions or suspensions of their salts, oxides or hydroxides by reaction with aqueous hydrogen peroxide solution and drying them.
In Doetsch et al., GB 2,048,842 (=DE 29 18 137), a concentrated alkali or alkaline-earth oxide or hydroxide suspension is reacted with aqueous hydrogen peroxide solution and subsequently is subjected to spray-drying.
It is known from Doetsch et al., U.S. Pat. No. 4,427,644 (=DE 30 25 682), to react a solid, anhydrous or hydrated oxide or hydroxide directly with hydrogen peroxide in an intensive mixer, and to dry the reaction product in a short-retention-time dryer.
Doetsch et al., U.S. Pat. No. 6,193,776 (=DE 196 50 686), describes a process for the preparation of homogenous Ca/Mg peroxides by reacting an aqueous suspension of calcium hydroxide and magnesium oxide and/or magnesium hydroxide with a hydrogen peroxide solution and subsequent drying of the resulting calcium/magnesium peroxide.
It is likewise known to delay the slow release of the oxygen by coating the peroxides.
In accordance with Gago, U.S. Pat. No. 4,293,426, calcium peroxide particles are granulated in the presence of water-insoluble compounds. Organic polymers or inorganic compounds are used here as water-insoluble compounds. These compounds have a solubility in water of less than 1% and a melting point of at least 50xc2x0 C. Coating of the peroxide particles in this case takes place in a known manner during the granulation process.
It is the object of the invention to provide metal peroxides which are stable in a moist medium, which greatly reduces the release of oxygen therefrom.
This and other objects are achieved in accordance with the present invention by providing a metal peroxide coated with a coating agent comprising at least one silicone compound selected from the group consisting of siliconates, hydrogen-polysiloxanes, siloxanes, silicone resin emulsions, silane/siloxane emulsions, silane/siloxane mixtures, silicone microemulsion concentrates, and silicic acid ester mixtures.
In accordance with a further aspect of the invention, the objects are achieved by providing a process for coating a metal peroxide comprising providing a suspension of the metal peroxide, while stirring the suspension introducing up to 10% by weight (relative to solids content of the suspension) of at least one silicone compound selected from the group consisting of siliconates, hydrogen-polysiloxanes, siloxanes, silicone resin emulsions, silane/siloxane emulsions, silane/siloxane mixtures, silicone microemulsion concentrates, and silicic acid ester mixtures, and drying the resulting coated metal peroxide.
Another aspect of the invention includes a method of bioremediating soil comprising treating the soil with an effective bioremediating amount of a coated metal peroxide as described above.
The invention involves hydrophobizing the metal peroxide particles by adding a suitable coating agent to a metal peroxide suspension before drying.
The metal peroxide suspension is prepared analogously to the methods described in DE 29 18 137, DE 30 25 682 and DE 196 50 686 by reacting the corresponding metal oxides and/or hydroxides with hydrogen peroxide. The coating agent is mixed directly into the resulting peroxide-containing suspension. The peroxide suspension supplied with the coating agent is dried in a short-retention-time dryer, e.g. spray-dryer. In contrast to U.S. Pat. No. 4,293,426, this ensures that the grain size of the metal peroxides is maintained. No agglomerates are formed. The coating agent directly surrounds the primary grains. Suitable coating agents for use in the invention include silicone compounds selected from the group consisting of siliconates, hydrogen/polysiloxane emulsions, siloxanes, siloxanes on support material, silicone resin emulsions, silane/siloxane mixtures, silicone microemulsion concentrates and silicic acid ester mixtures. In accordance with the invention, coating agent in an amount of up to 10% by weight (relative to the solids content of the suspension) is introduced with stirring into the metal peroxide suspension.
In one embodiment of the invention, a silicone microemulsion concentrate based on silane/siloxane is used as coating agent.
In another embodiment of the invention, silicone microemulsion concentrate composed of silane/siloxane is introduced into the metal peroxide suspension with stirring in a quantity of up to 10% by weight, preferably in a quantity of up to 2% by weight, relative to the solid product.
As used herein, the term xe2x80x9csilicone microemulsion concentrate based on silane/siloxanexe2x80x9d is understood to refer to a water-dilutable silane/siloxane synthetic resin mixture, preferably an alkyl silicone resin with alkoxy groups, in particular a partially neutralized amino-functional polydimethylsiloxane.
Preferably alkaline-earth oxides and alkaline-earth hydroxides, in particular calcium hydroxide or calcium oxide, magnesium oxide or magnesium hydroxide, are used as metal salts. Zinc oxide is likewise suitable for preparing the peroxides according to the invention.
Optionally, small amounts of the additives and/or stabilizers usually used in the preparation of inorganic peroxygen compounds may additionally be added to the metal peroxide solution. Examples of useful additives include, inter alia, waterglass or conventional active oxygen stabilizers such as commercially available phosphonic acids and salts thereof.
The hydrophobized metal peroxides coated according to the invention are dried in conventional manner by evaporation of the water with simultaneous drying of the reaction product in a short-retention-time dryer. Examples of suitable short-retention-time dryrs include spray-dryers and spin-flash dryers.
The coated hydrophobized metal peroxides may be formulated in a conventional manner depending on the intended use.
It has been found that the metal peroxides coated according to the invention have a considerably delayed release of oxygen compared with the known, untreated products. Owing to the delayed release of oxygen, the peroxides according to the invention are particularly suited to supporting the aerobic process of bioremediation and thus making soil purification more effective.
As a further positive effect, it has been discovered that alkaline earth peroxides hydrophobized according to the invention have improved flow behavior. Owing to their improved flow behavior, these products can, for example, be introduced into the soil very simply and without major technical outlay for the bioremediation of soils.